Hi Jane,
This shouldn't be too difficult to remove. I assume it's the hatch insert that won't rotate.
Rotation is anticlockwise to remove. Try a suitably sized piece of wood and a mallet/hammer to 'punch' the cover anticlockwise. This should rotate the cover off. You might need a new replacement hatch cover which should come with a seal. Use silicone lubricant on the seal before installing the cover/lid. If you manage to completely destroy the cover, then the old one will unscrew and you can replace with new using sealant between the cover and boat.
Make sure you buy the correct size RWO hatch cover - I think it's 5" but check. You can buy just the O ring seal.
https://www.sailboats.co.uk/rwo-inspection-hatchesHope that helps.
Pete
SN1177